1. Field of the Invention
The present invention relates, in general, to a drive mechanism for FF-4WD (Front Engine, Front Drive & 4 Wheel Drive) automobiles and, more particularly, to a drive mechanism with a middle shaft being positioned at a side of the transmission, thus allowing a transfer case to be easily installed in an engine compartment without causing any interference between the transfer case and the cylinder block of an engine.
2. Description of the Prior Art
As well known to those skilled in the art, a typical drive mechanism for 4WD-type automobiles comprises a transmission (T/M) gear 1, a ring gear 2, a center differential (C/D) unit 4 and a front differential (F/D) unit 5. The T/M gear 1 transmits the rotating force of a transmission (T/M) B to a transfer case (T/F) C as shown in FIGS. 3, 4a and 4b. The ring gear 2 is rotated by the T/M gear 1 and rotates the C/D case 3. The C/D unit 4 is positioned at the right-hand section in the C/D case 3, while the F/D unit 5 is positioned at the left-hand section in the C/D case 3.
The drive mechanism also includes a viscous coupling 7, which controls the differential operation of the C/D unit 4. A sleeve 6 connects the viscous coupling 7 to the C/D unit 4, thus transmitting power from the C/D unit 4 to the coupling 7. Fixed to the viscous coupling 7 is a T/F drive gear 8, which transmits the power of the sleeve 6 to a T/F driven gear 9. The T/F driven gear 9 is fitted over a T/F axle 9'. The T/F axle 9' is rotated by the T/F drive gear 8, thus transmitting the drive power of the F/D unit 5 to a rear differential (R/D) unit (not shown).
The C/D unit 4 is comprised of a plurality of C/D pinion gears 4a and two C/D side gears. The C/D pinion gears 4a are carried by the C/D case 3, while the two C/D side gears engage with and are rotated by the two C/D pinion gears 4a. In this case, the speed ratio between the two C/D side gears is variable in accordance with a rotating resistance acting on the front and rear wheels.
The two C/D side gears comprise right and left side gears 4b and 4c. The right C/D side gear 4b is connected to the sleeve 6. Meanwhile, the left C/D side gear 4c is connected to a middle shaft 4d, with an inner shaft 7a of the viscous coupling 7 being fitted over the middle shaft 4d.
The F/D unit 5 comprises an F/D case 5a, which is fixedly connected to the C/D left side gear 4c of the C/D unit 4, thus being rotatable along with the C/D left side gear 4c. The F/D case 5a also carries a plurality of F/D pinion gears 5b. The F/D unit 5 also includes two F/D side gears 5c, which engage with the F/D pinion gears 5b. The two F/D side gears 5c are thus rotated by the F/D pinion gears 5b at rotating speeds which may differ in accordance with a difference between the rotating resistances acting on two axles of the two F/D side gears 5c. That is, the speed ratio between the two F/D side gear 5c is variable. The F/D unit 5 further includes two front axles 5d, which are connected to and rotated by the two F/D side gears 5c.
In the operation of the above drive mechanism for 4WD-type automobiles, the drive power of an engine A is primarily transmitted to the transmission B, in which the rotating speed of the drive power is appropriately changed. The drive power, with a changed rotating speed, is, thereafter, appropriately distributed to four wheels of an automobile by the drive mechanism.
FIG. 5 is a block diagram showing the drive power transmission passage of the typical drive mechanism for distributing the drive power to the four wheels. As shown in the drawing, the drive power, with a changed rotating speed, is primarily transmitted to the ring gear 2 through the T/M gear 1, thus rotating the C/D case 3. Therefore, the C/D pinion gears 4a are rotated along with the C/D case 3 and rotate the two C/D side gears 4b and 4c.
In this case, the speed ratio between the two C/D side gears 4b and 4c is variable in accordance with the rotating resistances acting on the front and rear wheels (not shown). That is, when the rotating resistance acting on the two front wheels is different from that of the rear wheels, the C/D unit 4 performs a differential operation, with the C/D pinion gears 4a of the C/D unit 4 being rotated and revolved and making the two front wheels have a rotating speed, which is different from that of the rear wheels.
Since the F/D case 5a is integrated with the left side gear 4c of the C/D unit 4 as described above, the rotating force of the left side gear 4c is transmitted to the F/D case 5a, thus rotating the F/D case 5a along with the F/D pinion gears 5b. The two F/D side gears 5c are thus rotated by the F/D pinion gears 5b. The drive power in the form of the rotating force of the F/D side gears 5c is, thereafter, transmitted to the left and right front wheels through the two front axles 5d, thus rotating the two front wheels.
In such a case, when the rotating resistance acting on the left front wheel is different from that of the right front wheel, the F/D unit 5 performs a differential operation, in which the F/D pinion gears 5a are rotated and revolved and make the rotating speed of one front axle 5d for the left front wheel different from that of the other front axle 5d for the right front wheel.
The rotating force of the C/D right side gear 4b is transmitted to the sleeve 6, thus rotating the sleeve 6 along with the T/F drive gear 8. The T/F drive gear 8 rotates the T/F driven gear 9, thus allowing the drive power in the form of the rotating force of the T/F driven gear 9 to be transmitted to the R/D unit (not shown) through the T/F axle 9' or the center shaft of the T/F driven gear 9. The R/D unit appropriately distributes the drive power to the left and right rear wheels, thus rotating the two rear wheels.
During such an operation of the drive mechanism, the rotating force of the C/D left side gear 4c is transmitted to the middle shaft 4d, thus rotating the middle shaft 4d along with the inner shaft 7a of the viscous coupling 7. Meanwhile, the rotating force of the C/D right side gear 4b is transmitted to the sleeve 6, thus rotating the sleeve 6 along with the housing 7b of the viscous coupling 7. When the rotating speed of the front wheels is equal to that of the rear wheels, the inner and outer plates 7c and 7d of the viscous coupling 7 are rotated at the same rotating speed.
Meanwhile, when the rotating speed of the front wheels is different from that of the rear wheels, the inner and outer plates 7c and 7d of the viscous coupling 7 are rotated at different rotating speeds. However, the coupling 7 in the above state reduces the rotating speed difference between the two plates 7c and 7d using its viscosity, thus limiting the differential operation of the C/D unit 4.
However, such a typical drive mechanism for FF-4WD automobiles is problematic in that it is very difficult to install the transfer case C in an engine compartment since both the middle shaft 4d and the sleeve 6 of the drive mechanism causes an interference between the transfer case C and the cylinder block (not shown) of an engine during installation of the transfer case C.